Gas chromatography is an analytical technique which is widely used for analyzing volatile compounds in complex samples, given its high separation power as well as the existence of highly sensitive detectors.
However, a traditional drawback of gas chromatography is that it requires a sample preparation step which is usually laborious and involves the use of large amounts of solvents that are usually toxic for the analyst and hazardous for the environment. This sample preparation step further forms the greatest source of errors in the entire analytical process.
Liquid chromatography is mainly used when the compounds to be analyzed are thermolabile or are not volatile, because it has a lower separation power and the detectors are generally less sensitive. However, compared to gas chromatography, it has the advantage that “dirtier” samples can be analyzed, which allows simplifying or even eliminating the sample preparation step.
The use of the direct coupling of high performance liquid chromatography and gas chromatography is very useful for analyzing complex mixtures. The use of this type of “multidimensional system” allows combining the use of liquid chromatography as a sample preparation technique with the use of gas chromatography to obtain a highly sensitive detection (Grob, K., “On-Line Coupled LC-GC”, Hüthig, Heidelberg, Germany, 1991; Mondello, L., Dugo, G., Bartle, K D., “On-line Microbore HPLC-CGC for Food and Water analysis. A Review”, Journal of Microcolumn Separations, 1996, 8, 275-310). It is thus possible to have analysis methods that do not require the use of conventional sample preparation processes, which apart from being laborious and not very reliable, have the great drawback of requiring the use of relatively high volumes of contaminating organic solvents. In other words, traditional sample preparation can be substituted with a liquid chromatography step, in which a fluid is generated from which a fraction containing the elements or solutes to be detected/analyzed is selected. This fraction is later subjected to a gas chromatography phase.
A problematic aspect in relation to the use of the direct coupling of liquid chromatography and gas chromatography relates to the features of the interface necessary to make said coupling possible. Two essentially different systems in which the operating parameters are substantially different must be made compatible.
The initially developed interfaces only allowed the use of normal phase in the pre-separation performed by liquid chromatography because the physical and chemical characteristics of the solvents used in normal phase (small vapor volume per unit of liquid volume, low surface tension, etc.) facilitate this coupling.
Different interfaces (“autosampler”, “on-column”, “loop-type”) which allow the direct coupling of normal phase liquid chromatography and gas chromatography have been designed and used (Grob, K., “Development of the Transfer Techniques for On-Line HPLC-CGC”, Journal of Chromatography A 1995, 703, 265-76; Vreuls, J. J., de Jong, G. J., Ghijsen, R. T., Brinkman, U. A. Th., “LC Coupled On-Line with GC: State of the Art”, Journal of the Association of Official Analytical Chemistry International 1994, 77, 306-27).
However, in many cases it is necessary to turn to the use of reverse phase in the liquid chromatography step in order to achieve a certain separation. In fact, most of the analytical applications in which only liquid chromatography is involved are carried out in reverse phase. Therefore, the extension of the field of applicability of the direct coupling of liquid chromatography and gas chromatography requires the development of suitable interfaces for carrying out direct coupling between reversed-phase liquid chromatography and gas chromatography (Señorans, F. J., Villén, J, Tabera, J., Herraiz, M. “Simplex Optimization of the Direct Analysis of Free Sterols in Sunflower Oil by On-Line Coupled RPLC-GC”, Journal of Agricultural and Food Chemistry 1998, 46, 1022-26; Villén, J, Blanch, G. P., Ruiz of the Castillo, M. L., Herraiz, M., “Rapid Analysis of Free Erythrodiol and Uvaol in Olive Oils by Coupled RPLC-GC”, Journal of Agricultural and Food Chemistry 1998, 46, 1027-31).
With this aim in mind, several systems have been proposed over the last few years (“retention gap”, “concurrent solvent evaporation”, “open tubular trap”, etc) (Grob, K., “Development of the Transfer Techniques for On-Line HPLC-CGC”, Journal of Chromatography A 1995, 703, 265-76; Vreuls, J. J., of Jong, G. J., Ghijsen, R. T., Brinkman, U. A. Th. “LC Coupled On-Line with GC: State of the Art”, Journal of the Association of Official Analytical Chemistry International 1994, 77, 306-27) although the limitations involved in using polar eluents (fundamentally the high volumes of vaporization produced during transfer and the difficulty of suitably focusing the chromatographic band) have prevented the development of an interface meeting the required conditions as regards simplicity, reliability, versatility and possibility of automation.
Patent application WO-A-99/061127, corresponding to U.S. Pat. No. 6,402,947-B1, the content of which is included herein as a reference, describes an interface device for the direct coupling of liquid chromatography and gas chromatography, designed based on a basic scheme of a PTV (programmed temperature vaporizer) injector which has been modified so that is can be used for the direct coupling of liquid chromatography in normal phase or reverse phase, and gas chromatography, and for the introduction of high volumes in gas chromatography (i.e. of sample volumes which are greater than those usually introduced in gas chromatography, which allows increasing the sensitivity of the analysis). This interface device comprises an outer body with a first end part, a second end part, an intermediate section between said end parts, and an inner cavity divided into a first inner chamber and a second inner chamber, as well as an inner tube arranged in said inner cavity. The inner tube has a first section arranged in the first inner chamber, a second section arranged in the second inner chamber, the first section ending in a first end with a first opening and the second section ending in a second end, as well as an inner channel for housing an adsorbent material trapped between two inorganic wool “plugs”. The second section of the inner tube is communicated with a waste conduit. The injector body also comprises a dividing element surrounding the inner tube and dividing the inner cavity into the first inner chamber and the second inner chamber. The device further comprises a system for selecting a liquid chromatography fraction and leading it to the inner tube by means of a first duct with a free end penetrating into the tube through the first end of the tube, and a first valve connected to the opposite end of the first duct, and a discharge system for discharging the liquid chromatography fraction into the inner tube.
This discharge system is designed to prevent the liquid chromatography fraction from entering the gas chromatography column when the device operates in the adsorption mode, which column communicates with the inner tube by means of a second duct which also penetrates into the inner tube through the first end of said inner tube. To that end, the free end of the first duct ending inside the inner tube is located at a first distance from the adsorbent material, and the free end of the second duct which also penetrates into the inner tube is located at a second distance from the adsorbent material, the second distance being greater than said first distance, whereby the possibility of part of the liquid fraction from the liquid chromatography injected in the inner tube in the adsorption phase entering the second tube, i.e. in the tube corresponding to the gas chromatography column, is reduced. Since the first tube and the second tube are located at the same end of the inner tube, and since the waste tube is located at the second end of the inner tube, the “flow” in the adsorption phase goes from the first end to the second end, whereby, due to the fact that the free end of the second duct is “withdrawn” in an “upstream” direction with respect to the free end of the first duct, it is very difficult for a part of the fraction entering the inner tube from the first tube to pass to the second tube when the system operates in an adsorption phase, as described in WO-A-99/061127 (and U.S. Pat. No. 6,402,947-B1).
The configuration described in WO-A-99/061127 can have several drawbacks, for example as regards the flexibility it offers for analysis equipment designers. The arrangement with the supply duct for supplying the liquid fraction from the liquid chromatography system located at the same end of the inner tube as the gas column represents a certain limitation while designing complete equipment with its hydraulic, pneumatic and electronic components including the heating system and the detector or detectors associated to the gas chromatography column.
In addition, the fact that the transfer capillary traverses the gas chromatograph oven, as described in WO-A-99/061127 (and U.S. Pat. No. 6,402,947-B1), makes it very recommendable for the oven to be maintained at a temperature below the boiling point of the solvents which are transferred to the inner tube during the transfer, as described in the applications developed up until now (Pérez, M., Alario, J., Vázquez, A. and Villén, J., “On-Line Reversed Phase LC-GC by using the New TOTAD (Through Oven Transfer Adsorption Desorption) Interface: Application to Parathion Residue Analysis”, Journal of Microcolumn Separations 1999, 11, 582-589; Pérez, M., Alario, J., Vázquez, A. and Villén, J., “Pesticide Residue Analysis by Off-Line SPE and On-Line Reversed Phase LC-GC using the New TOTAD (Through Oven Transfer Adsorption Desorption) Interface”, Analytical Chemistry 2000, 72, 846-852; Alario, J., Pérez, M., Vázquez, A. and Villén, J., “Very Large Volume Sampling of Water in GC using the TOTAD (Through Oven Transfer Adsorption Desorption) Interface for Pesticide Residue Analysis”, Journal of Chromatography Science 2001, 39, 65-69; Sanchez, R., Vázquez, A. M., Riquelme, D. and Villén, J., “Direct Analysis of Pesticide Residues in Olive Oil by On-Line Reversed Phase Liquid Chromatography-Gas Chromatography using an Automated Through Oven Transfer Adsorption Desorption(TOTAD) Interface”, Journal of Agriculture and Food Chemistry 2003, 51, 6098-6102; Sanchez, R., Vázquez, A. M., Andini, J. C. and Villén, J., “Automated Multiresidue Analysis of Pesticides in Olive Oil by On-Line Reversed Phase Liquid Chromatography-Gas Chromatography using the Through Oven Transfer Adsorption Desorption Interface”, Journal of Chromatography A 2004, 1029, 167-172; Sanchez, R., Cortés, J. M., Villén, J. and Vázquez, A. M., “Determination of Organophosphorus and Triazine Pesticides in Olive Oil by On-Line Reversed-Phase Liquid Chromatography-Gas Chromatography with a Nitrogen-Phosphorus Detector Using an Automated Through Oven Transfer Adsorption-Desorption Interface”, Journal of the Association of Official Analytical Chemistry International 2005, 88, 1255-1260).
Once the transfer is over, it is necessary to heat the gas chromatograph oven until the temperature necessary to start the chromatographic separation and the corresponding analysis, and once the latter has ended, it is necessary to cool the gas chromatograph oven until the temperature that it must have during the transfer. These oven temperature changes involve a time loss (normally from 10 to 15 minutes over a total of 40 to 80 minutes which the analysis lasts, according to the specific application) reducing the capacity of the system to carry out determinations, and which could be prevented if the transfer capillary did not traverse the gas chromatograph oven.
However, this arrangement has been considered necessary to prevent the liquid fraction from passing to the gas chromatography column during the adsorption phase.